Displays using OLEDs are self-luminous displays, in which a fluorescent organic compound is excited to emit light. Such a self-luminous display has advantages in that it can be driven at a low voltage, while having a thin structure. Since this display also has features such as a wide viewing angle and a rapid response speed, it is being highlighted as a next-generation display candidate capable of solving problems incurred in liquid crystal displays (LCDs). Also, this display is being highlighted as a next-generation flat panel display in that it can have a picture quality equivalent to or better than that of thin film transistor (TFT) LCDs in the case of a medium or smaller size, and it is advantageous in terms of price competitiveness because the manufacturing process thereof is simple.
Hereinafter, the operation principle of the display, which uses OLEDs, will be described in brief. As electric power is supplied into an OLED, current flows through the OLED in accordance with movement of electrons. Specifically, electrons (positive charge) at the side of an anode are moved to a light emitting layer in accordance with assistance of an electron transporting layer. On the other hand, holes (electron deficiencies, negative charge) at the side of a cathode are moved to the light emitting layer in accordance with an assistance of a hole transporting layer. As a result, the electrons and holes are recombined in the light emitting layer, which is made of an organic material, thereby producing excitons having high energy. When the energy of excitons is reduced to a base level, light is emitted. The color of the emitted light is determined, depending on the kind of the organic material forming the light emitting layer. Using organic materials capable of emitting red (R), green (G), and blue (B), respectively, it is possible to realize a full-color display. Thus, the above-mentioned display uses self-luminous organic materials, as compared to LCDs, which simply use a function of switching on/off pixels.
OLED display devices, which are used as thin film display devices, have been advanced from a passive matrix pixel arrangement to an active matrix pixel arrangement, as in commercially available LCDs, which are currently widely used. Although passive matrix type OLED display devices have advantages of a simple arrangement and application of correct data to each pixel, they have a drawback in that it is difficult to implement large-size and high definition displays. For this reason, development of active matrix type OLED display devices is actively underway.
Now, a drive circuit of a conventional active matrix type OLED display device will be described with reference to FIG. 1.
FIG. 1 is a schematic view illustrating an OLED drive circuit, which includes general active matrix type pixel circuits.
Referring to FIG. 1, the OLED drive circuit includes a matrix arrangement of a plurality of scanning lines X1, X2, X3, . . . for selecting or deselecting pixels 30 at intervals of a predetermined scanning cycle (for example, a frame period according to the NTSC Standard), and a plurality of data lines Y1, Y2, Y3, . . . for supplying luminance information to drive the pixels 30. The pixels 30 are formed at respective intersections of the matrix arrangement. Each pixel is constituted by a pixel circuit.
The scanning lines X1, X2, X3, . . . are connected to a scanning line drive circuit 20, whereas the data lines Y1, Y2, Y3, . . . are connected to a data line drive circuit 10. A desired image can be displayed by sequentially selecting the scanning lines X1, X2, X3, . . . by the scanning line drive circuit 20, applying a voltage corresponding to the luminance information applied to an associated one of the data lines Y1, Y2, Y3, . . . to each pixel of the selected scanning line through the associated data line, and repeating the voltage application for all pixels of the sequentially selected scanning lines. In accordance with a drive circuit of a passive matrix type OLED display device, the light emitting element of each pixel emits light only at a moment when the light emitting element is selected. On the other hand, in the drive circuit of an active matrix type OLED display device, the light emitting element of each pixel continuously emits light even after the completion of the application of luminance information thereto. Accordingly, the active matrix type OLED display device is advantageous in terms of high definition display in a large size screen because the light the drive current level of the light emitting element thereof is lowered, as compared to that of the passive matrix type OLED display device.
The operation of the drive circuit in the OLED display device including a plurality of pixels 30 will now be described in detail. In accordance with the drive operation of the drive current, the scanning line drive circuit 20 selects one scanning line XN from the scanning lines X1, X2, X3, . . . , and transmits a select signal to the selected scanning line XN, and the data line drive circuit 10 transmits data, that is, luminance information, to the pixels of the selected scanning line XN through the data lines Y1, Y2, Y3, . . . , respectively. Thereafter, the scanning line drive circuit 20 transmits a deselect signal to the selected scanning line XN. In this state, the scanning line drive circuit 20 selects the next scanning line XN+1, and then transmits a select signal to the selected next scanning line XN+1. As the select and deselect signals are sequentially transmitted to the scanning lines, transmission of data can be repeatedly and sequentially achieved. Accordingly, the drive circuit of the OLED display device can display a desired image.
FIG. 2 is a circuit diagram illustrating a pixel circuit included in the conventional drive circuit of the active matrix type OLED display device.
Referring to FIG. 2, the pixel circuit, which is adapted to drive one pixel 30, includes an OLED, first and second NMOS transistors T1 and T2, and a capacitor Cs. The first transistor T1 performs current control. The transistor T1 is connected at a source thereof to the OLED, while being connected at a drain thereof to a positive voltage source Vdd. The transistor T2 is connected at a gate thereof to the scanning line XN associated therewith, while being connected at a drain thereof to the data line YM associated therewith. The source of the transistor T2 is connected to both the gate of the transistor T1 and the capacitor Cs. The OLED is connected to a cathode thereof to a ground voltage source. Accordingly, the voltage of the data line YM is applied to the gate of the transistor T1 through the transistor T2, so as to control current flowing through the OLED.
When the transistor T2 receives, at the gate thereof, a select signal from the scanning line XN, it is turned on. At this time, the voltage corresponding to the luminance information applied to the data line YM from the data line drive circuit 10 is applied to the gate of the transistor T1 via the transistor T2. The luminance information voltage is also stored in the capacitor Cs. As a result, the gate voltage of the transistor T1 is stably maintained by the capacitor Cs even for one frame period, in which the transistor T2 is maintained in an OFF state thereof by a deselect signal applied to the scanning line XN. Accordingly, the current flowing in the OLED via the transistor T1 is constantly maintained.
Since the current flowing through the OLED corresponds to the current flowing from the drain of the transistor T1 to the source thereof in the above-mentioned conventional pixel circuit, this current can be controlled by the gate voltage of the transistor T1. However, this current may be different from a desired current due to a degradation in the characteristics of the transistor T1 caused by a non-uniformity of the characteristics of the transistor T1 or a prolonged operation of the transistor T1.
TFTs, which are used in display devices, are positive elements easily meeting the requirement of high definition and large-size display. However, such TFTs may have a threshold voltage deviation of several hundred mV even though they are formed on the same substrate. In some cases, there may be a threshold voltage deviation of 1V or more. In such a case, there may be a problem in that, although the same signal voltage Vw is inputted to TFTs of pixels, the amounts of current flowing through respective OLEDs of the pixels may be different from each other greatly beyond an allowable range when respective TFTs of the pixels have different threshold voltages. In this case, it is impossible to expect a good display quality. Such a threshold voltage difference is inevitably present between different manufacturing routes or different products, even though it may not be large. For this reason, it is necessary to determine the data line potential causing a desired drive current to flow through the OLED, based on parameters, which may be determined to have different values for different products. However, this method is impractical for mass production of displays.
Furthermore, the TFTs may involve a great variation in initial threshold voltage value due to a degradation in characteristics caused by ambient temperature or prolonged use. In this case, the display quality or brightness may severely vary during use of the display device. For this reason, the life of the display device may be abruptly reduced. However, it is very difficult to provide a measure capable of solving this problem.